In a conventional electrophotographic recording apparatus, a magnetic roller is used as a developing roller or a cleaning roller. A magnet roller typically comprises a cylindrical permanent magnet, such as a resin-bonded magnet, provided with a plurality of magnetic poles on the outer surface thereof and a shaft secured to the cylindrical permanent magnet.
Multipole magnetic rollers are typically made of sintered, discrete magnets mounted on a fluted shaft, or of a compounded mixture of magnetic powder and resin to form a so called resin-bonded magnet. Resin-bonded magnets are primarily used when weight reduction of the developer roll is desired or when a unique shape is desired. When a cylindrical permanent magnet is a resin-bonded magnet, the magnetic roller is produced typically by (1) blending a ferromagnetic powder (usually ferrite powder) and a polymer material (usually rubber or plastic) to prepare a mixture, (2) charging the mixture into a cavity of an injection-molding die while applying a magnetic force thereto, (3) cooling and solidifying the cylindrical resin-bonded magnet in the die, (4) removing a fully magnetized integral body composed of the cylindrical, resin-bonded magnet and shaft, and (5) magnetizing the integral body in appropriate anisotropic directions to form magnetic poles on the cylindrical resin-bonded magnet, thereby obtaining the complete magnetic roller.
Production of magnetic rollers typically causes mold wear as a result of the high ferric content of the ferrite/polymer mixtures and as a result of high pressures used during molding. In addition, to be useful, a magnetic roller must have the correct combination of physical, morphological and magnetic properties. These properties are greatly affected by straightness of the magnetic roller, and the straightness is degraded in direct relationship to molded “stress” in the roller. Molded in “stress” is directly related to higher molding pressure.
The cost of the magnetic rollers is typically a function of raw material, processing and handling costs. Thereby, lower material usage, shorter processing times and/or less handling may potentially reduce the cost of each magnetic roller. In addition, a reduction in the amount of material allows faster cooling of the magnetic roller and improved cycle times allowing more magnetic rollers to be produced from each individual mold. Cycle time describes the amount of time to produce a magnetic roller in an injection-molding machine.
Typically, manufacture of magnetic rollers requires a high clamp pressure and a high molding pressure. Formulations which allow reductions in clamp pressures and/or molding pressures, further allow magnetic roller to be made on smaller, less expensive presses. In addition, current conventional manufacturing techniques often incur high scrap rates from restarts after cleaning of flash from the mold. The generation of flash is a function of manufacturing pressure, wherein lower pressure during production of a magnetic roller typically reduces flash and accordingly increases throughput. Additionally, lower molding pressures reduce molded in stress and result in more dimensionally accurate ports.
Thus, the need exists for improved magnetic rollers and/or improved manufacture of magnetic rollers.
Foam molding is known and the foregoing advantages are known to result from foam molding, particularly because internal pressures in the mold are significantly reduced by the action of gases formed during molding